Ultra-thin layers of rust generate electricity from flowing water

Ultra-thin layers of rust generate electricity from flowing water
Credit: Morteza Akhnia/Unsplash

There are many ways to generate electricity—batteries, solar panels, wind turbines, and hydroelectric dams, to name a few examples… and now, there's rust.

New research conducted by scientists at Caltech and Northwestern University shows that thin films of rust—iron oxide—can generate electricity when saltwater flows over them. These films represent an entirely new way of generating electricity and could be used to develop new forms of sustainable power production.

Interactions between metal compounds and saltwater often generate electricity, but this is usually the result of a chemical reaction in which one or more compounds are converted to new compounds. Reactions like these are what is at work inside batteries.

In contrast, the phenomenon discovered by Tom Miller, Caltech professor of chemistry, and Franz Geiger, Dow Professor of Chemistry at Northwestern, does not involve chemical reactions, but rather converts the kinetic energy of flowing saltwater into electricity.

The phenomenon, the electrokinetic effect, has been observed before in thin films of graphene—sheets of carbon atoms arranged in a hexagonal lattice—and it is remarkably efficient. The effect is around 30 percent efficient at converting into electricity. For reference, the best are only about 20 percent efficient.

"A similar effect has been seen in some other materials. You can take a drop of saltwater and drag it across graphene and see some electricity generated," Miller says.

However, it is difficult to fabricate graphene films and scale them up to usable sizes. The iron oxide films discovered by Miller and Geiger are relatively easy to produce and scalable to larger sizes, Miller says.

"It's basically just rust on iron, so it's pretty easy to make in large areas," Miller says. "This is a more robust implementation of the thing seen in graphene."

Though rust will form on iron alloys on its own, the team needed to ensure it formed in a consistently thin layer. To do that, they used a process called (PVD), which turns normally solid materials, in this case iron oxide, into a vapor that condenses on a desired surface. PVD allowed them to create an layer 10 nanometers thick, about 10 thousand times thinner than a .

When they took that rust-coated iron and flowed saltwater solutions of varying concentrations over it, they found that it generated several tens of millivolts and several microamps per cm-2.

"For perspective, plates having an area of 10 square meters each would generate a few kilowatt-hours—enough for a standard US home," Miller says. "Of course, less demanding applications, including low-power devices in remote locations, are more promising in the near term."

The mechanism behind the electricity generation is complex, involving ion adsorption and desorption, but it essentially works like this: The ions present in saltwater attract electrons in the iron beneath the layer of rust. As the saltwater flows, so do those ions, and through that attractive force, they drag the electrons in the along with them, generating an electrical current.

Miller says this effect could be useful in specific scenarios where there are moving saline solutions, like in the ocean or the human body.

"For example, tidal energy, or things bobbing in the ocean, like buoys, could be used for passive electrical energy conversion," he says. "You have flowing in your veins in periodic pulses. That could be used to generate for powering implants."

The paper describing their findings, titled "Energy Conversion via Metal Nanolayers," appears in the July 29 issue of the Proceedings of the National Academy of Sciences.

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More information: Mavis D. Boamah et al. Energy conversion via metal nanolayers, Proceedings of the National Academy of Sciences (2019). DOI: 10.1073/pnas.1906601116
Citation: Ultra-thin layers of rust generate electricity from flowing water (2019, July 30) retrieved 19 September 2019 from https://phys.org/news/2019-07-ultra-thin-layers-rust-electricity.html
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Jul 30, 2019
What are "kilowatts per hour"?

Jul 30, 2019
What are "kilowatts per hour"?
I think in this case 'per' means 'for each' instead of '/'
"The kilowatt hour (symbolized kW⋅h as per SI) is a composite unit of energy equivalent to one kilowatt (1 kW) of power sustained for one hour. One watt is equal to 1 J/s. One kilowatt hour is 3.6 megajoules, which is the amount of energy converted if work is done at an average rate of one thousand watts for one hour." (ref)

Jul 30, 2019
oops, edit wasn't late, double post

Jul 30, 2019
The math doesn't add up. Several tens of millivolts at several microamps would be on the order of 100 nanowatts. That's per cm², so each square meter generates roughly a milliwatt. So to generate a kilowatt would take roughly 1,000,000 m², which would require roughly 50,000 two-sided plates of 10 m² each...

Jul 30, 2019
Yeah, agreed; I checked your calculations. Hmmm. I'll see if I can find an open access version of the paper. This might just be a dopey writer of the article; let's see what the paper says if we can.

Jul 31, 2019
Figured it out; the flow rates they used were quite small. They're extrapolating, I think.

Jul 31, 2019

Da Schneib
Jul 30, 2019
Because teh vul siensetis are alweys rong, amirite @SkieLie?

Jul 31, 2019
"The effect is around 30 percent efficient at converting kinetic energy into electricity. For reference, the best solar panels are only about 20 percent efficient."

This article employs an improper apples and oranges comparison, given the comparison of photovoltaic (PV) efficiencies (photons) with kinetic efficiencies (molecules).

Yes, PV efficiencies top out at about 20%, but turbogenerators can easily have an efficiency of 80-90%. IMHO, a 30% electrical efficiency using moving water is nothing to brag about.

Especially when the 30% efficiency, based on an expensive physical vapor deposition (PVD) process, assumes that ZERO rust will EVER deposit on top of the expensive PVD nanolayer.



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